Light receiving elements, which are typical elements of optical semiconductor devices, convert an optical signal to an electrical signal and are used in various fields. In particular, in the field of optical discs, such as CDs (Compact Discs) and DVDs (Digital Versatile Discs), light receiving elements are important as key devices in optical pickup devices that read and write signals recorded on optical discs. In recent years, in response to the demand for higher performance and higher degree of integration, so-called optoelectronic integrated circuits (OEICs), in which a photodiode serving as a light receiving element and various electronic elements, such as a bipolar transistor, a resistor, and a capacitor, are integrated together on the same substrate, have been put to practical use. Also, optical discs are required to become faster and smaller and compatible with various kinds of discs. And in OEICs, it is required that a light receiving element having high light-receiving sensitivity, high speed and low noise characteristics and a bipolar transistor having high speed and low noise characteristics be integrated together. Recently, in particular, in accordance with the demand for an increase in the capacity of optical discs, the commercialization of HD-DVDs and Blu-ray Discs (BDs) that employ a blue semiconductor laser (having a wavelength of 405 nm) as a light source has begun, and hence there is a demand for OEICs that have high speed, high light-receiving sensitivity and low noise characteristics in the short wavelength region corresponding to the blue semiconductor laser.
An optical semiconductor device according to a first conventional example will be described below.
FIG. 7 is a cross-sectional view schematically illustrating the structure of the optical semiconductor device, which is an OEIC, according to the first conventional example. In FIG. 7, the optical semiconductor device in which a pin (p-intrinsic-n) photodiode as a light receiving element and an NPN transistor as a bipolar transistor are formed on a p-type silicon substrate is illustrated as an example.
As shown in FIG. 7, the optical semiconductor device of the first conventional example includes: a silicon substrate 101 containing a low concentration of a p-type impurity; an NPN transistor 103 formed on the silicon substrate 101; a photodiode 102 formed on the silicon substrate 101; and a light absorbing element 104 formed in a region on the silicon substrate 101 between the NPN transistor 103 and the photodiode 102. The conventional optical semiconductor device also includes: a first p-type buried layer 105 formed on the silicon substrate 101 and containing a high concentration of a p-type impurity; a p-type epitaxial layer 106 formed on the first p-type buried layer 105 and containing a low concentration of a p-type impurity; an n-type epitaxial layer 107 formed on the p-type epitaxial layer 106; a LOCOS (local oxidation of silicon) isolation layer 108 formed on the n-type epitaxial layer 107 and isolating the light absorbing element 104 and the photodiode 102 from each other; and an insulating film 109 formed on the n-type epitaxial layer 107 and on the LOCOS isolation layer 108.
The photodiode 102 includes: a part of the above-mentioned p-type epitaxial layer 106; a part of an n-type cathode layer 110 composed of a part of the n-type epitaxial layer 107; a part of a cathode contact layer 111 formed on the cathode layer 110 and containing a high concentration of an n-type impurity; an anode buried layer 113 containing a high concentration of a p-type impurity; an anode contact layer 114 formed on the anode buried layer 113 and containing a high concentration of a p-type impurity; a first cathode electrode 112 formed on the cathode contact layer 111; and a first anode electrode 115 formed on the anode contact layer 114.
The NPN transistor 103 includes: a collector buried layer 116 formed between the p-type epitaxial layer 106 and the n-type epitaxial layer 107 and containing a high concentration of an n-type impurity; a collector contact layer 117 formed on the collector buried layer 116 and containing a high concentration of an n-type impurity; a p-type base layer 119 formed in a region in the n-type epitaxial layer 107 located above the collector buried layer 116; an emitter layer 121 formed in the base layer 119 and containing a high concentration of an n-type impurity; a collector electrode 118 formed on the collector contact layer 117; a base electrode 120 formed on the base layer 119; and an emitter electrode 122 formed on the emitter layer 121.
The light absorbing element 104 includes: a part of the p-type epitaxial layer 106; a part of the cathode layer 110 formed on the p-type epitaxial layer 106; a part of the cathode contact layer 111 formed on the cathode layer 110; the p-type anode buried layer 113 formed on the p-type epitaxial layer 106; the p-type anode contact layer 114 formed on the anode buried layer 113; a second cathode electrode 123 formed on the cathode contact layer 111; and a second anode electrode 124 formed on the anode contact layer 114.
Operation of the conventional optical semiconductor device thus configured will be described below.
When there is incident light on the upper surface of the photodiode 102, the cathode layer 110, and the p-type epitaxial layer 106 which serves as the anode absorb the light, thereby generating electron-hole pairs. At this time, if a reverse bias V1 is applied to the photodiode 102, a depletion layer expands in the p-type epitaxial layer 106 having a low impurity concentration. The electrons and holes of electron-hole pairs generated in the vicinity of the depletion layer are separated from each other by diffusion and drift, so that the electrons reach the cathode contact layer 111, and the holes reach the anode buried layer 113. The carriers are drawn out from the first cathode electrode 112 and from the first anode electrode 115 as a photocurrent. This photocurrent is subjected to amplification and signal processing performed by electronic circuits composed of the NPN transistor 103 and resistance elements, capacitive elements, etc. that are integrated together on the silicon substrate 101, and then the photocurrent is output so as to become an optical-disc record or reproduction signal.
In the first p-type buried layer 105 that contains a higher concentration of a p-type impurity than the silicon substrate 101, a potential barrier is formed. Since the silicon substrate 101 is not depleted, carriers generated in the silicon substrate 101 move by diffusion, however, due to this potential barrier, those carriers recombine with carriers of opposite signs in the first p-type buried layer 105 and do not reach the p-type epitaxial layer 106. Furthermore, if the impurity concentration in the p-type epitaxial layer 106 is lowered to the point that the p-type epitaxial layer 106 is completely depleted, a drift current, which is a high-speed component, will become dominant in the photocurrent. A diffusion current, which is a low-speed component, will make almost no contribution to the photocurrent, thereby allowing the photodiode 102 to operate at high speed. Moreover, the above-mentioned potential barrier formed by the first p-type buried layer 105 prevents the carriers generated in the silicon substrate 101 from reaching the NPN transistor 103 as well, whereby malfunctions and noise components in the transistor are suppressed.
If the carriers generated in the p-type epitaxial layer 106 serving as the anode reach the NPN transistor 103, those carriers will become a collector current component and thus cause malfunctions and noise components in the circuit. In particular, when light is not sufficiently focused on the photodiode 102 and is incident outwardly (of the photodiode 102), many carriers will be generated in part of the p-type epitaxial layer 106 located in the boundary region between the photodiode 102 and the NPN transistor 103, causing this unfavorable phenomenon to become more predominant.
In contrast, in the optical semiconductor device according to the first conventional example, the light absorbing element 104 is formed between the photodiode 102 and the NPN transistor 103, and a reverse bias is applied between the second cathode electrode 123 and the second anode electrode 124 during operation. Therefore, a photodiode is formed by the cathode layer 110 in the light absorbing element 104 and the p-type epitaxial layer 106 serving as the anode, so that escaping carriers are absorbed in the cathode contact layer 111 and in the anode contact layer 114. If the wires of the second cathode electrode 123 and of the second anode electrode 124 are not connected with the wires of the photodiode 102 and of the NPN transistor 103, unnecessary components are prevented from entering the signal processing section, thereby reduce noise.
Next, an optical semiconductor device according to a second conventional example for suppressing escaping carriers will be described.
FIG. 8 is a cross-sectional view schematically illustrating the structure of the optical semiconductor device of the second conventional example. In this example, a light absorbing element is not provided, and a photodiode 102 has a different structure from that of the first conventional example.
The optical semiconductor device of the second conventional example includes: an n-type buried layer 126 formed on a silicon substrate 101 and containing a high concentration of an n-type impurity; a p-type buried layer 125 formed on the n-type buried layer 126 and containing a high concentration of a p-type impurity; an n-type cathode layer 110 formed on the p-type buried layer 125; an n-type cathode contact layer 111 formed on the cathode layer 110; an anode contact layer 114 formed on the n-type buried layer 126 and containing a high concentration of a p-type impurity; an n-type contact layer 127 formed on the n-type buried layer 126; a first cathode electrode 112 formed on the cathode contact layer 111; a first anode electrode 115 formed on the anode contact layer 114; and a dummy cathode electrode 128 formed on the n-type contact layer 127.
In this structure, a photodiode is formed by the PN junction between an n-type epitaxial layer 107 (the cathode layer 110) and the p-type buried layer 125. In addition, PN junctions are also formed between the p-type buried layer 125 and the n-type buried layer 126 and between the n-type buried layer 126 and the silicon substrate 101.
Moreover, a reverse bias V1 is applied between the first cathode electrode 112 and the first anode electrode 115, and a reverse bias V2 is applied between the dummy cathode electrode 128 and the first anode electrode 115. Therefore, carriers generated by light absorbed by the silicon substrate 101 are absorbed by the dummy photodiode and move through the n-type buried layer 126 and the n-type contact layer 127 to the dummy cathode electrode 128, thus making no contribution to the photocurrent. Consequently, a slow diffusion current component is suppressed, thereby enabling high-speed operation. Also, if the reverse biases are set so as to satisfy V1<V2, it is also possible to suppress carriers that escape from the photodiode 102 to the NPN transistor 103.